1. Field of the Invention
The present invention relates to a fuel cell, a method of manufacturing the same, and a fuel cell stack including the same.
2. Description of the Related Art
As mobile apparatuses such as portable personal computers, mobile phones, and portable game machines have improved in their performance and functions, there has been an increasing demand for an increase in their battery capacity.
For instance, in recent years and continuing, the mobile phones tend to consume more and more power due to larger display screens, the addition of a moving image display function, and the provision of W-CDMA next-generation mobile phone services.
Currently, lithium secondary cells are widely used in these apparatuses. In these lithium secondary cells, however, more than 90% of the storage capacity of their electrode material is already being utilized. Further, it is believed that for safety reasons, the energy density of lithium secondary cells cannot be increased beyond approximately 450 Wh/L. Therefore, no significant increase can be expected in their unit storage capacity.
Under these circumstances, efforts have been dedicated to the research and development of small-size fuel cells as next-generation energy devices that can be expected to have extremely high unit storage capacity.
The direct methanol fuel cell (DMFC) is one of various types of small-size fuel cells.
The DMFC generates hydrogen by decomposing methanol used as fuel in the presence of a catalyst provided to a fuel electrode, and causes the generated hydrogen to react with oxygen, thereby generating power. The DMFC can realize energy density approximately ten times that of lithium ion-based cells. Further, unlike methanol reforming-type fuel cells, the DMFC does not require a reformer for decomposing methanol. Therefore, the DMFC can be easily reduced in size and weight, and can be used for a long period.
A description is given below, with reference to FIG. 1, of a conventional DMFC as disclosed in Japanese Laid-Open Patent Application No. 2001-283892. FIG. 1 is a sectional view of a cell pack composed of a plurality of DMFCs 1. The layer components of the cell pack are shown spaced for convenience of description.
Each DMFC 1 includes a proton exchange membrane (PEM) 2 and a fuel electrode (a catalyst layer) 3a and an air electrode (a catalyst layer) 3b joined to the respective sides of the PEM 2. Each of the fuel electrode 3a and the air electrode 3b is formed of carbon paper and holds a catalyst thereon. Metal-mesh collectors (collector plates) 4a and 4b are provided to the fuel electrode 3a and the air electrode 3b, respectively, on the side opposite to the PEM 2. A separate conductive member (not shown in the drawing) is electrically connected to each of the collectors 4a and 4b to be extended outward. Reference numeral 5 denotes channels for airflow.
In this case, the above-described DMFCs 1 are connected in series by an electric connection member 6. The connected DMFCs 1 are sandwiched between upper and lower end plates 7 and 8. The end plates 7 and 8 are fastened to each other by screws (not shown in the drawing). Thus, the DMFCs 1 are integrated into the single cell pack. A fuel inlet 7a and a fuel outlet are formed in the end plate 7. The fuel outlet, which is positioned behind the fuel inlet 7a in FIG. 1, is not graphically represented. An air supply hole 8a is formed in the end plate 8. Reference numeral 9 denotes a flow prevention member for preventing fuel from entering the air electrode 3b side.
If the conductive members of a constant thickness connected to the collectors 4a and 4b or the collectors 4a and 4b themselves are directly extended outside the seal part of the main body of the DMFC 1, fuel leakage may be caused due to sealing deficiency. Therefore, fastening members formed of conductive material are attached through the seal part, or actually, through a housing covering the main body of the DMFC 1 including the seal part, to the conductive members or the collectors 4a and 4b provided inside the seal part. The fastening members function as extension electrodes. Each fastening member is also attached with a seal structure.
In the DMFC 1, a methanol aqueous solution is supplied as fuel to the fuel electrode 3a. The solution comes into contact with the catalyst of the fuel electrode 3a so that methanol and water in the solution react with each other to be converted to protons (hydrogen ions) and carbon dioxide. The generated protons pass through the PEM 2 to reach the air electrode 3b. Then, the protons react with oxygen included in the air supplied to the air electrode 3b in the presence of its catalyst, and are converted to water. Electrons are generated by the reaction in the fuel electrode 3a, and are consumed by the reaction in the air electrode 3b. These electrons flow from the collector 4a into the collector 4b through a load (not shown in the drawing). As a result, power is generated to be supplied to the load.